1. Field of the Invention
The present invention generally relates to a magnetoresistive element, a magnetic head, and a magnetic memory apparatus, and more particularly to a magnetoresistive element having a CPP (Current-Perpendicular-To-Plane) structure that allows a sense current to flow in a direction perpendicular to a spin valve film, a magnetic head, and a magnetic memory apparatus thereof.
2. Description of the Related Art
In recent years, a magnetoresistive element has been used in a magnetic head of a magnetic recording apparatus as a reproduction element for reproducing information recorded in a magnetic recording medium. The magnetoresistive element is mainly provided with a spin valve film having high magnetic field sensitivity for accommodating high recording densities. The spin valve film comprises layers which include a fixed magnetization layer in which the direction of magnetization is fixed in predetermined direction, a non-magnetic layer, and a free magnetic layer in which the direction of magnetization changes in correspondence with the direction or the strength of a leaking magnetic field of the magnetic recording medium. The value of the electric resistance of the spin valve film changes in correspondence with the angle that is formed by the magnetization of the fixed magnetization layer and the magnetization of the free magnetic layer. The magnetoresistive element reproduces bits recorded to the magnetic recording medium by applying a sense current of a predetermined value to the spin valve film and detecting the change of electric resistance (change of voltage).
Conventionally, a CIP (Current-In-Plane) structure, which allows a sense current to flow in an in-plane direction of the spin valve film, is used for the magnetoresistive element. However, in order to record with higher recording density, an increase in linear recording density and in track density of the magnetic recording medium is required. Accordingly, reduction of the width of the magnetoresistive element corresponding to the track width of the magnetic recording medium and the height of the magnetoresistive element (depth of magnetoresistive element), that is, reduction of the cross-sectional area of the magnetoresistive element is required. In such a case, the current density of the sense current becomes excessively large when the CIP structure is used for the magnetoresistive element. This leads to deterioration of performance of the magnetoresistive element due to, for example, migration of materials included in the spin valve film. In order to prevent such deterioration of performance, the amount of sense current is to be reduced. However, such reduction of sense current causes reduction in the amount of detected voltage change corresponding to the change of magnetic resistance, that is, reduction in reproduction output, to thereby result in deterioration of S/N ratio (signal to noise ratio).
Therefore, as a reproduction element of the next generation, vigorous research is being made for a magnetoresistive element using a CCP (Current-Perpendicular-To-Plane) structure that allows a sense current to flow in a direction perpendicular to the spin valve film.
In a case where the magnetoresistive element uses the CPP structure, the value of element resistance decreases since the thickness of the spin valve film is thin. Accordingly, a sufficient reproduction output cannot be obtained merely by using a material which is the same as that of the magnetoresistive element having the CIP structure since the amount of change of the magnetic resistance is small (RA). Therefore, in order to obtain a sufficient recording output for the magnetoresistive element having the CPP structure, a large change amount of the magnetic resistance per unit area (RA) is desired to be provided to the spin valve film.
In order to increase RA, a magnetoresistive element having a thin insertion layer (e.g. Co—Fe alloy or a Co—Ni alloy) provided between a free magnetic layer and a non-magnetic layer is proposed (See, for example, Japanese Laid-Open Patent Application No. 2003-60263). In this magnetoresistive element, the RA is expected to be increased by generating a spin dependent bulk scattering of the free magnetic layer and a spin dependent interface scattering of the thin insertion layer.
However, in using the materials of the thin insertion layer and the configuration of the spin valve film as shown in Japanese Laid-Open Patent Application No. 2003-60263, the increase of RA is insufficient.
Furthermore, in obtaining higher recording density, it is difficult to increase RA while maintaining balance with the low coercive force characteristic of the free magnetic layer. That is, there is a problem where sensitivity of the magnetoresistive element deteriorates as the coercive force of the free magnetic layer increases.